1. Field of the Invention
The present disclosure relates to an optical scanning apparatus which scans a target surface by light, and an image forming device including the optical scanning apparatus provided therein.
2. Description of the Related Art
Image forming devices, such as digital multi-functional peripherals or laser printers, are provided with an optical scanning apparatus. In the optical scanning apparatus, a light beam emitted from a light source is deflected by an optical deflector having a rotary polygon mirror, so that a photoconductor drum surface is scanned by the deflected light beam.
Generally, an image forming device including plural photoconductor drums uses an optical scanning apparatus having two scanning lenses arranged in two opposed positions that confront opposite sides of the optical deflector respectively. In the following, the optical scanning apparatus of this type will be called an opposed scanning type optical scanning apparatus.
The optical scanning apparatus may include a sync detecting sensor arranged at a predetermined position where a light beam prior to a write start time is received, in order to equalize write start positions for plural scanning lines in a main scanning direction on the photoconductor drum surface. In the following, an output signal of the sync detecting sensor will be called a sync detection signal.
In the opposed scanning type optical scanning apparatus, a single sync detecting sensor may be arranged at a position confronting one of the photoconductor drums. In this case, a false sync signal (or pseudo sync signal) for the other photoconductor drums is generated based on the sync detection signal output from the sync detecting sensor confronting the one of the photoconductor drums.
Mirror surfaces of the rotary polygon mirror in the above-mentioned optical deflector are formed by cutting. However, at a result of a normal cutting process, the angle between two adjacent ones of the mirror surfaces may not be uniform. In such a case, when the photoconductor drum surface is scanned by the light beam deflected on a mirror surface different from the mirror surface from which the sync detection signal is obtained, if a write start timing of an image is determined from a false sync signal generated based on the sync detection signal, the write start position of the image will deviate from the correct position. If the precision of cutting is improved, the irregularity of the angle may be reduced. However, the process cost will increase conversely.
For example, Japanese Patent No. 4,393,133 discloses an image forming device which is arranged to include a detection unit arranged at a position corresponding to a first light emitting device to detect a laser beam from the first light emitting device scanned by a rotary polygon mirror. A signal generating unit generates a horizontal sync signal for determining the timing to form an electrostatic latent image in a main scanning direction on an image support object with the laser beam from the first light emitting device according to a result of the detection by the detection unit. A measuring unit measures an interval of the detection times that the laser beams of the first light emitting device scanned by the faces of the polygon mirror are detected by the detection unit sequentially. A timing determination unit determines the timing to form the electrostatic latent image on the image support object by a laser beam of a second light emitting device scanned by a face of the polygon mirror different from the face by which the laser beam of the first light emitting device is scanned, based on the detection of the laser beam of the first light emitting device by the detection unit when generating the horizontal sync signal, and the interval measured by the measuring part, without detecting the laser beam of the second light emitting device to be scanned by the polygon mirror.
Japanese Laid-Open Patent Publication No. 2003-185952 discloses an optical scanning apparatus in which two or more of plural scanning units share a polygonal deflector. The two or more scanning units use the light beams deflected by different deflection surfaces of the polygonal deflector. A single write start position detecting unit is arranged to detect the light beams from the different deflection surfaces of the polygonal deflector. The write start timing for the scanning units to the target surface is determined by using an output signal of this write start position detecting part.
Moreover, Japanese Laid-Open Patent Publication No. 2004-102276, Japanese Laid-Open Patent Publication No. 2006-305780, Japanese Patent No. 3,773,884, and Japanese Laid-Open Patent Publication No. 2011-011504 disclose the related technology similar to that of the above-mentioned related art documents.
However, in these years, the requirements for increasingly high image quality of image forming devices are present, and there has been a problem that the image forming device disclosed in Japanese Patent No. 4,393,133 and the image forming device using the optical scanning apparatus disclosed in Japanese Laid-Open Patent Publication No. 2003-185952 have difficulty in satisfying the requirements.
The inventors of the present application have examined the image quality of an image forming device including an optical scanning apparatus in which a false sync signal is generated to determine a write start timing, and have discovered that the image quality is affected by the rotation irregularity of the rotary polygon mirror.
Moreover, as known conventionally, the error (the face-by-face error) for the deflection reflective surfaces of the polygon mirror is also a problem that affects the image quality.
The main factor of the face-by-face error is the variations in the distance from the rotation axis of the polygon mirror to the deflection reflective surfaces of the optical deflector (or eccentricity of the polygon mirror and profile irregularities of the faces of the polygon mirror).
One of the causes of the irregularity of the write end position is that the scanning irregularity is produced when two or more light sources are used. The main factor of this scanning irregularity is that there is a difference in the oscillation wavelength of the light sources, and the scanning speed is varied according to the chromatic aberration of the scanning optical system.